[#289] Refactor Morley.Nettest errors types

Description

Problem: At the moment, expectFailure accepts UnexpectedBalance and UnexpectedClientSuccess as args, even though it doesn't make sense to say:

expectBalance n `expectFailure` NettestUnexpectedBalance x

Solution: Split NettestFailure into two types: one for errors thrown by the interpreter/client (that the user can expect to happen and handle), and another for errors thrown by cleveland when an assertion fails (which should not be caught, only displayed in the console).

Related issue(s)

Resolves #289 (closed)

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